Production of a semiconductor device involves a step of forming an electroconductive film on the surface of a wafer to form a wiring layer by photolithography, etching etc., a step of forming an interlaminar insulating film on the wiring layer, etc., and an uneven surface made of an electroconductive material such as metal and an insulating material is generated on the surface of a wafer by these steps. In recent years, processing for fine wiring and multilayer wiring is advancing for the purpose of higher integration of semiconductor integrated circuits, and accordingly techniques of planarizing an uneven surface of a wafer have become important.
As the method of planarizing an uneven surface of a wafer, a CMP method is generally used. CMP is a technique wherein while the surface of a wafer to be polished is pressed against a polishing surface of a polishing pad, the surface of the wafer is polished with an abrasive in the form of slurry having abrasive grains dispersed therein (hereinafter, referred to as slurry).
As shown in FIG. 1, a polishing apparatus used generally in CMP is provided for example with a polishing platen 2 for supporting a polishing pad 1, a supporting stand (polishing head) 5 for supporting a polished material (wafer) 4, a backing material for uniformly pressurizing a wafer, and a mechanism of feeding an abrasive. The polishing pad 1 is fitted with the polishing platen 2 for example via a double-coated tape. The polishing platen 2 and the supporting stand 5 are provided with rotating shafts 6 and 7 respectively and are arranged such that the polishing pad 1 and the polished material 4, both of which are supported by them, are opposed to each other. The supporting stand 5 is provided with a pressurizing mechanism for pushing the polished material 4 against the polishing pad 1.
When such CMP is conducted, there is a problem of judging the planarity of wafer surface. That is, the point in time when desired surface properties or planar state are reached should be detected. With respect to the thickness of an oxide film, polishing speed etc., the polishing treatment of a test wafer has been conducted by periodically treating the wafer, and after the results are confirmed, a wafer serving as a product is subjected to polishing treatment.
In this method, however, the treatment time of a test wafer and the cost for the treatment are wasteful, and a test wafer and a product wafer not subjected to processing are different in polishing results due to a loading effect unique to CMP, and accurate prediction of processing results is difficult without actual processing of the product wafer.
Accordingly, there is need in recent years for a method capable of in situ detection of the point in time when desired surface properties and thickness are attained at the time of CMP processing, in order to solve the problem described above. In such detection, various methods are used. The detection means proposed at present include:
(1) a method of detecting torque wherein the coefficient of friction between a wafer and a pad is detected as a change of the rotational torque of a wafer-keeping head and a platen (U.S. Pat. No. 5,069,002),
(2) an electrostatic capacity method of detecting the thickness of an insulating film remaining on a wafer (U.S. Pat. No. 5,081,421),
(3) an optical method wherein a film thickness monitoring mechanism by a laser light is integrated in a rotating platen (JP-A 9-7985 and JP-A 9-36072),
(4) a vibrational analysis method of analyzing a frequency spectrum obtained from a vibration or acceleration sensor attached to a head or spindle,
(5) a detection method by applying a built-in differential transformer in a head,
(6) a method wherein the heat of friction between a wafer and a polishing pad and the heat of reaction between slurry and a material to be polished are measured by an infrared radiation thermometer (U.S. Pat. No. 5,196,353),
(7) a method of measuring the thickness of a polished material by measuring the transmission time of supersonic waves (JP-A 55-106769 and JP-A 7-135190), and
(8) a method of measuring the sheet resistance of a metallic film on the surface of a wafer (U.S. Pat. No. 5,559,428). At present, the method (1) is often used, but the method (3) comes to be used mainly from the viewpoint of measurement accuracy and spatial resolution in non-constant measurement.
The optical detection means as the method (3) is specifically a method of detecting the endpoint of polishing by irradiating a wafer via a polishing pad through a window (light-transmitting region) with a light beam, and monitoring an interference signal generated by reflection of the light beam (FIG. 12).
At present, a He—Ne laser light having a wavelength light in the vicinity of 600 nm and a white light using a halogen lamp having a wavelength light in 380 to 800 nm is generally used.
In such method, the endpoint is determined by knowing an approximate depth of surface unevenness by monitoring a change in the thickness of a surface layer of a wafer. When such change in thickness becomes equal to the thickness of unevenness, the CMP process is finished. As a method of detecting the endpoint of polishing by such optical means and a polishing pad used in the method, various methods and polishing pads have been proposed.
A polishing pad having, as least a part thereof, a solid and uniform transparent polymer sheet passing a light of wavelengths of 190 to 3500 nm therethrough is disclosed (Japanese Patent Application National Publication (Laid-Open) No. 11-512977). Further, a polishing pad having a stepped transparent plug inserted into it is disclosed (JP-A 9-7985). A polishing pad having a transparent plug on the same surface as a polishing surface is disclosed (JP-A 10-83977). Further, a polishing pad wherein a light-permeable member comprises a water-insoluble matrix material and water-soluble particles dispersed in the water-insoluble matrix material and the light transmittance thereof at 400 to 800 nm is 0.1% or more is disclosed (JP-A 2002-324769 and JP-A 2002-324770). It is disclosed that a window for endpoint detection is used in any of the polishing pad.
As described above, a He—Ne laser light and a white light using a halogen lamp are used as the light beam, and when the white light is used, there is an advantage that the light of various wavelengths can be applied onto a wafer, and many profiles of the surface of the wafer can be obtained. When this white light is used as the light beam, detection accuracy should be increased in a broad wavelength range. In high integration and micronization in production of semiconductors in the future, the wiring width of an integrated circuit is expected to be further decreased, for which highly accurate optical endpoint detection is necessary, but the conventional window for endpoint detection does not have sufficiently satisfactory accuracy in a broad wavelength range.
The object of a first invention is to provide a polishing pad enabling highly accurate optical detection of endpoint during polishing and thus having excellent polishing characteristics (surface uniformity etc.) and a method of producing a semiconductor device by using the polishing pad.
An object of a second invention is to provide a polishing pad enabling highly accurate optical detection of endpoint during polishing and particularly preferably usable in a polishing apparatus using a He—Ne laser light or a semiconductor laser having a transmission wavelength in the vicinity of 600 to 700 nm and thus having excellent polishing characteristics (surface uniformity etc.). Another object of the second invention is to provide a polishing pad which can be easily and inexpensively produced, as well as a method of producing a semiconductor device by using the polishing pad.
On one hand, the window (light-transmitting region) described in the patent specifications supra is long in the circumferential direction of the polishing pad or is circular, as shown in FIGS. 2 and 3. In the case of the window having the shape described above, however, the window contacts intensively with only a part of a wafer during polishing the wafer, and thus there is a problem that uneven polishing occurs between a portion contacting with the window and a portion not contacting with the window. There is also a problem that an obtainable polishing profile is that of a limited portion contacting with the window.
The object of a third invention is to provide a polishing pad enabling highly accurate optical detection of endpoint during polishing, thus having excellent polishing characteristics (particularly in-plane uniformity etc.) and capable of giving the polishing profile of a wide area of a wafer, as well as a method of producing a semiconductor device by using the polishing pad.